1. Field of the Invention
This invention relates to supported metal oxide catalysts for use in pollution control systems. Such structures of this type generally eliminate substantially all the NO.sub.x and SO.sub.x contained within the gas streams typically exhausted from a coal burning furnace.
2. Description of the Related Art
Selective catalytic reduction, hereinafter referred to as SCR, has been used to remove NO.sub.x from gas streams exhausted from a coal furnace by injecting a reducing agent, usually ammonia (NH.sub.3), in the presence of an oxide catalyst, such as vanadium oxide. Vanadium oxide supported on inert substrates containing titanium dioxide (TiO.sub.2) has been found to catalyze the reduction of NO to nitrogen and water and also react with SO.sub.2 under the same conditions. While the dual purpose makes vanadium catalysts desirable for pollution control, these prior pollution control systems suffer from at least two drawbacks.
First, a reducing agent such as ammonia must be injected into the system in order to decompose the NO.sub.x. However, not all the ammonia injected into the system reacts with the NO.sub.x. The unreacted ammonia, which can reach concentrations as high as 50 ppm in the exhaust gas, is usually expelled into the atmosphere typically as ammonia leakage and this is highly undesirable.
Second, the gas streams leaving the coal furnace contain water in the form of stream. The steam adsorbs onto the substrate and deactivates the catalyst with a typical catalyst being deactivated by 10-50% within one day of operating when the steam concentration in the gas stream is greater than 5% v/v. Deactivation by water occurs because the TiO.sub.2 substrate has a high affinity for water, i.e., is hydrophilic. Therefore, a more advantageous system, then, would be presented if such amounts of unreacted ammonia and deactivation of the catalyst could be reduced.
It is apparent from the above that there exists a need in the art for a pollution control system which reduces the amount of NO.sub.x and SO.sub.x pollution being exhausted into the atmosphere, and which at least equals the NO.sub.x and SO.sub.x pollution control of the prior art systems, particularly those of the SCR type, but which at the same time substantially reduces the ammonia leakage and the water or steam deactivation of the catalyst. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.